Abstract

A method is developed to demodulate (velocity correct) Fourier transform spectrometer data that are taken with an analog to digital converter that digitizes equally spaced in time. This method makes it possible to use simple low-cost, high-resolution audio digitizers to record high-quality data without the need for an event timer or quadrature laser hardware and makes it possible to use a metrology laser of any wavelength. The reduced parts count and simple implementation make it an attractive alternative in space-based applications when compared to previous methods such as the Brault algorithm.

Figures (6)

Synthetic quadrature phase detector using a synthetic reference derived from the average laser fringe frequency to demodulate the laser fringe signal and velocity correct the spectral signal by resampling the spectral signal evenly in space.

Derived phase error and displacement as a function of sample number. The phase error in the left graph is the deviation from the phase derived from the average frequency—(f(t)−fa)t. The right graph is the phase error plus fat.

Derived fringe/sample rate obtained from differencing the displacement with respect to sample number. This is directly proportional to the velocity. The left graph shows the profile over the entire scan, and the right graph shows the fine detail.

Fourier transform of a laser interforogram before and after processing. Since we can resample at any rate we can even velocity correct the reference metrology laser. The above correction was done by resampling at 3 times per fringe, which is the approximate original data acquisition rate. Since the data are captured evenly spaced in time and corrected evenly space in distance, the units along the x axis must be different.

Fourier transform of a spectral interferogram before and after processing. Before correction the absorption spectra are barely visible. After correction one can clearly see the characteristic CO absorption lines. Since the data are captured evenly spaced in time and corrected evenly space in distance, the units along the x axis must be different.